1. Field of the Invention
The present invention generally relates to feedthroughs that provide electrical paths for implantable medical devices. The feedthroughs hermetically connect the components, including the electrical power and control circuitry, housed inside the casing of an implantable medical device with the conductors delivering the therapy to the body being assisted. More particularly, the present invention helps prevent electromagnetic interference (hereinafter EMI) with critical electrical signals in these devices.
2. Prior Art
Medical devices, including implantable cardiac pacemakers and implantable cardiac defibrillators currently in use for correcting cardiac abnormalities, are adversely affected by spurious EMI. Spurious EMI is highly undesirable because it can interfere with proper functioning of the implanted medical device, either, by inhibiting a proper response or by causing an improper one, and can otherwise cause the device to unpredictably malfunction. Spurious EMI signals are emitted from such common sources as television transmitters, cell phones, cell towers, and anti-theft detection devices. Thus, all of these EMI emitters can cause significant problems for individuals with implantable medical devices.
The structure of a pacemaker or a defibrillator device typically includes a casing as a housing for a pulse generator and associated circuitry, and a battery that serves as a power supply. One or more conductive lead wires extend from the pulse generator circuit in the interior of the device and pass through the casing where they connect via a medical lead to an electrode surgically attached to an appropriate location in the heart. A feedthrough allows the lead wires to hermetically pass from the interior of the device, through the casing and but to the medical lead connected to the heart. The typical feedthrough comprises a ferrule mountable on the device casing with an insulator positioned within the ferrule. The lead wires pass through the insulator from the interior of the device to the exterior in a non-conductive manner. This is possible because the lead wires are electrically isolated from the metallic device casing.
Shunting the lead wires to a ground wire or pin by a filter capacitor connected between them essentially eliminates stray EMI. There may be more than one ground wire. Typically, one capacitor is positioned between each lead wire and at least one of the ground pins. When used with a multipin feedthrough, these capacitors are often built as a monolithic structure or array and are referred to as an internally grounded feedthrough. If the array is in the form of a right circular cylinder, it is designated a discoidal capacitor.
However, prior art feedthrough devices are not without problems. For example, if the internally grounded feedthrough is to effectively filter EMI, the ground pin or pins must be electrically connected to the ferrule. Spot welding or brazing the ground pin or pins to the ferrule typically accomplishes this. Spot welding and brazing are time consuming, increase the number of components required, increase the number of manufacturing steps, and result in a ground that is not centrally located.
Another method for grounding the feedthrough is to braze a metallic member to one wire passing through a hole in the ceramic insulator. The opposite end of the metallic member is then brazed to an insulator-ferrule braze joint. Problems associated with this construction include undesirably increasing the number of components necessary for the feedthrough, increasing the need for complicated manufacturing processes to accomplish the required brazing, and decreasing production yields.
Yet another past attempt at grounding a feedthrough is to furnish the insulator with a metallized ground strip down its side. Gold is then flowed down the ground strip to the insulator-ferrule braze joint. This design presents significant fixturing problems, has high variability, and often results in a very thin gold strip with excessive resistivity, thus making it unsuitable for many implantable device applications.
Additionally, ground members that are brazed directly to the ferrule can develop hermetic failures after completion of device assembly if the brazing process is not controlled and dissolution of titanium in the gold braze is excessive. Braze failures occur when the titanium to gold ratio is too high. This causes brittleness in areas of the braze fillet when the lead wire is subjected to bending strain or thermal stresses.
Then, there are feedthroughs that centralize the ground pin or pins where they are directly attached to the ferrule. However, these designs require multiple insulators, multiple components, increased seal lengths, increased feedthrough size, increased production time, and, ultimately, increased expense.
Thus, there is a need for an internally grounded feedthrough, preferably filtered, that comprises at least one centralized ground wire or pin with minimal spacing between the at least one lead wire and the at least one ground pin. The feedthrough needs to have a high degree of durability and be of a highly manufacturable design with a minimal number of components. This helps lower production costs and susceptibility to process variations while retaining ductile braze bonds.
The present multipin feedthrough comprises a ferrule surrounding an insulator supporting one or more lead wires and at least one ground pin. The insulator defines a channel cutout extending from a first insulator side to a channel cutout bottom part way through the thickness of the insulator and in communication with the ferrule. An attached filter capacitor shunts electromagnetic interference from the lead wire to the ground pin, and the ground pin is in electrical communication with the ferrule by way of a ferrule-ground pin braze joint formed in the channel cutout. This structure overcomes problems associated with prior art filter feedthroughs by shunting undesirable EMI directly from the ground pin to the ferrule along the ferrule-ground pin braze joint.